Micro-electromechanical systems (MEMS) are micro-mechanical transducers. MEMS actuators can take an electrical signal and convert it into a mechanical signal (e.g. movement). Conversely, MEMS sensors can take a mechanical signal (e.g. displacement or frequency) and can transfer that signal to an electrical or optical carrier. MEMS chemical sensors generally consist of a cantilever beam or bridge that is coated with a chemo-selective polymer, whose displacement and/or resonant frequency can be continuously monitored. Changes in the environment's chemical composition can affect the mechanical behavior of the MEMS sensor. Therefore, the MEMS sensor can provide a convenient means for transducing signals arising from the presence of chemical agents in the environment into measurable electrical or optical signals. FIG. 1A is a schematic of a prior art microbridge resonator and mechanical resonance spectrum.
For example, in one approach (i.e., static mode), upon chemical exposure, the chemo-selective polymer can selectively adsorb an analyte and the induced strain can result in beam bending, or displacement. A second approach (i.e., dynamic mode) can rely on exciting the mechanical resonances of the MEMS sensor. Upon chemical exposure, the polymer can selectively adsorbs the analyte and the resulting mass loading and induced strain shifts the resonance frequencies. FIGS. 1A and 1B are schematics of a prior art microbridge resonator and mechanical resonance spectrum, reflecting the downshift of mechanical resonance upon chemical vapor exposure, in accordance with the dynamic mode approach discussed.
Some advantages of a MEMS sensor can include: (1) high displacement sensitivity—simple (noninterferometric) optical readout enables displacement measurements of the order of 1 nm, (2) small mass, which makes the devices highly sensitive to any adsorbed chemical and hence mass loading—attogram mass changes have been measured, (3) small size—13×13 cantilever arrays with each element with area A<100 μm×100 μm have been realized, and (4) high selectivity—sensor arrays with multiple chemo-selective coatings enable detection of multiple analytes.
Various readout methods for interrogating the mechanical behavior of a MEMS sensor have been demonstrated. For example, one method takes advantage of piezo-resistance; the resistance across a micromechanical resonator changes with displacement and induced strain. The resistance change is small and generally requires two cantilever piezoresistors in a Wheatstone bridge configuration along with a differential amplifier to measure resonance. Similarly, in another method, a capacitive readout requires integrated amplifiers to convert a capacitance modulation into a measurable resonance frequency. Finally, the displacement amplitude for capacitive readout is generally in the tens of nanometers, which may require significant power to excite and measure the mechanical resonances.
Optical MEMS Sensors with Off-Chip Detection
Compared to electrical methods, optical methods can be completely passive and allow for remote readout via free-space or optical fibers. For example, a common optical detection method can utilize a position-sensitive photodetector in conjunction with a laser beam focused on the tip of the resonator. As the cantilever bends or oscillates, the reflected laser beam is deflected, modulated, and read out by the photodetector. FIG. 2A is a schematic of an optical displacement readout using a position sensitive photodetector. While this method is reasonably sensitive (displacements of one nanometer can be measured) and potentially enables remote readout, it typically requires accurate alignment between the probe laser and micro-resonator and cannot be packaged compactly due to the requirement of a long optical lever arm.
Another optical readout method can utilize interferometry to measure small displacements. For example, a Michelson interferometric readout setup can take advantage of interference between a reference laser beam and a second beam that is reflected off a cantilever that has been coated with a reflective metal. FIG. 2B is a schematic of an optical readout using an off-chip Michelson interferometer. Interference between the two beams can enable measurement of displacements many times smaller than the wavelength of light, λ, used in the experiment. In fact, displacements of the order of 10 picometers can be measured using interferometric techniques. However, a potential limitation with this approach is the need for precision alignment of the interferometer with the cantilever. Furthermore, it is not possible to interrogate multiple sensors (cantilevers) using a single off-chip interferometer. Multiple sensors require multiple interferometers, making it virtually impossible to develop large sensor arrays utilizing this particular optical readout method.
Optical MEMS Sensors with On-Chip Detection
In order to develop large sensor arrays, an on-chip optical readout is needed. By incorporating an optical waveguide onto the resonator, i.e. a cantilever waveguide, along with a second fixed waveguide, the device can be self-aligned during fabrication (provided there is no cantilever deflection due to intrinsic strain). This eliminates the need for accurate alignment required by the optical readout methods, as pictured in FIG. 2A and FIG. 2B. By sending laser light through the waveguides, any cantilever motion modulates the optical power coupled between the waveguides. FIG. 3A is a schematic representing a cantilever waveguide on-chip optical readout, in accordance with the prior art. Sub-nanometer displacement, mass loading, chemical sensing, and microfluidic flow rate measurements have been demonstrated with this approach. However, a potential drawback that still exists is the nonlinear response with waveguide displacement, which limits the sensitivity.
The highest displacement measurement resolution is obtained using interferometry, an approach that finds application in a variety of areas ranging from accelerometers to detection of gravity waves to mesoscopic quantum physics measurements. An external, off-chip, interferometric readout setup was described previously in relation to FIG. 2B. By making the micromechanical resonator reflective and coupling it to a second fixed mirror, an on-chip Fabry-Perot microcavity interferometer can be formed. FIG. 3B is a schematic representing an on-chip vertical-cavity Fabry-Perot interferometric readout, in accordance with the prior art. If the mirror reflectance is large, then the microcavity can have high finesse, and small displacements of the mirror can result in large changes in optical response. For example, displacements (e.g., <<1 nm) much smaller than the laser wavelength (e.g., λ≈1550 nm) can be measured. Also, the device is alignment free by design, allowing a simple setup. However, a drawback with this approach is the limited number of sensors that can be incorporated in a single-chip array. For example, previous MEMS chemical sensors with on-chip interferometric readout utilized a 1×4 array of optical microcavities vertical to the substrate. On the other hand, arrays with many (e.g., hundreds or more) of sensors may be difficult to achieve due to challenges in the interrogation of multiple vertical cavities with a single laser beam.